Well known is the apparition of St. Michael the Archangel (a. 494 or 530-40), as related in the Roman Breviary, 8 May, at his renowned sanctuary on Monte Gargano, where his original glory as patron in war was restored to him.
To his intercession the Lombards of Sipontum (Manfredonia) attributed their victory over the Greek Neapolitans, 8 May, 663.
In commemoration of this victory the church of Sipontum instituted a special feast in honor of the Archangel, on 8 May, which has spread over the entire Latin Church and is now called (since the time of Pius V) “Apparitio S. Michaelis”, although it originally did not commemorate the apparition, but the victory.
(cfr. Catholic Encyclopedia)
